Brazing is a method used for bonding metals, making the bond stronger than the metals. Brazing is also cost effective and serves a wide range of applications in industry today. However, it takes a special process for stainless steel brazing, to get the best results.
What is Stainless Steel?
The term “stainless steel” is a blanket term for several types of steel created to resist corrosion and rust. To make stainless steel, chrome and iron are alloyed along with other elements. Depending on the metals used, properties like increased strength and resistance to cold environments can be created. Metals like copper, titanium, nickel, and molybdenum, are sometimes added. In addition, nitrogen and carbon may also be used.
The most important properties for stainless steel concern resistance to rust. This had led to more than one grade of stainless steel, and there are 5 different types used today:
- Austenitic – this is the most common used type. It is made with nitrogen, nickel, and manganese. Materials like chromium are added to enhance corrosion resistance and this metal can be welded. However, it cannot be heat treated to harden and is subject to stress fractures under extreme conditions.
- Ferritic – carbon is used to create a structure similar to carbon steel.
- Duplex – this material is essentially half austenitic and half ferritic for higher resistance to stress and corrosion.
- Martensitic – uses higher levels of carbon for added strength
- PH – stands for precipitation hardening and gives stainless steel exceptional strength.
Stainless Steel Brazing
It is possible to braze most stainless steel types. However, grades containing niobium or titanium are not brazed. There are three basis methods for stainless steel brazing.
Flux
Flux is applied to the joints to be bonded. This is known as “in air brazing” and is achieved with low temperature heating methods. Depending on the application, a wide range of flux materials may be used.
Reduced Atmosphere
This method of stainless steel brazing has seen increased use over the past few decades since fuel injection has become common in the automotive field. Special filler material is used, creating an oxide free area to be bonded. This takes place in a controlled atmosphere with gases like oxygen, hydrogen, and water vapor, to prevent metal contamination.
Vacuum Method
Vacuum technology is used to create a very strong bond. The filler has a lower melting point than the stainless steel, and this type of stainless steel brazing is done in vacuum. It requires temperatures greater than one thousand degrees Celsius (1832 F).